[PATCH 06/91] perf stat: Improve runtime stat for interval mode

Arnaldo Carvalho de Melo
2020-05-06T15:23:07Z

From: Jin Yao <yao.jin@linux.intel.com>

For interval mode, the metric is printed after the '#' character if it
exists. But it's not calculated by the counts generated in this
interval.

See the following examples:

root@kbl-ppc:~# perf stat -M CPI -I1000 --interval-count 2
# time counts unit events
1.000422803 764,809 inst_retired.any # 2.9 CPI
1.000422803 2,234,932 cycles
2.001464585 1,960,061 inst_retired.any # 1.6 CPI
2.001464585 4,022,591 cycles

The second CPI should not be 1.6 (4,022,591/1,960,061 is 2.1)

root@kbl-ppc:~# perf stat -e cycles,instructions -I1000 --interval-count 2
# time counts unit events
1.000429493 2,869,311 cycles
1.000429493 816,875 instructions # 0.28 insn per cycle
2.001516426 9,260,973 cycles
2.001516426 5,250,634 instructions # 0.87 insn per cycle

The second 'insn per cycle' should not be 0.87 (5,250,634/9,260,973 is
0.57).

The current code uses a global variable 'rt_stat' for tracking and
updating the std dev of runtime stat. Unlike the counts, 'rt_stat' is not
reset for interval. While the counts are reset for interval.

perf_stat_process_counter()
{
if (config->interval)
init_stats(ps->res_stats);
}

So for interval mode, the 'rt_stat' variable should be reset too.

This patch resets 'rt_stat' before read_counters(), so the runtime stat
is only calculated by the counts generated in this interval.

With this patch:

root@kbl-ppc:~# perf stat -M CPI -I1000 --interval-count 2
# time counts unit events
1.000420924 2,408,818 inst_retired.any # 2.1 CPI
1.000420924 5,010,111 cycles
2.001448579 2,798,407 inst_retired.any # 1.6 CPI
2.001448579 4,599,861 cycles

root@kbl-ppc:~# perf stat -e cycles,instructions -I1000 --interval-count 2
# time counts unit events
1.000428555 2,769,714 cycles
1.000428555 774,462 instructions # 0.28 insn per cycle
2.001471562 3,595,904 cycles
2.001471562 1,243,703 instructions # 0.35 insn per cycle

Now the second 'insn per cycle' and CPI are calculated by the counts
generated in this interval.

Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Tested-By: Kajol Jain <kjain@linux.ibm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@intel.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200420145417.6864-1-yao.jin@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
tools/perf/Documentation/perf-stat.txt | 2 ++
tools/perf/builtin-stat.c | 1 +
2 files changed, 3 insertions(+)

diff --git a/tools/perf/Documentation/perf-stat.txt b/tools/perf/Documentation/perf-stat.txt
index 4d56586b2fb9..3fb5028aef08 100644
--- a/tools/perf/Documentation/perf-stat.txt
+++ b/tools/perf/Documentation/perf-stat.txt
@@ -176,6 +176,8 @@ Print count deltas every N milliseconds (minimum: 1ms)
The overhead percentage could be high in some cases, for instance with small, sub 100ms intervals. Use with caution.
example: 'perf stat -I 1000 -e cycles -a sleep 5'

+If the metric exists, it is calculated by the counts generated in this interval and the metric is printed after #.
+
--interval-count times::
Print count deltas for fixed number of times.
This option should be used together with "-I" option.
diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index 9207b6c45475..3f050d85c277 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -359,6 +359,7 @@ static void process_interval(void)
clock_gettime(CLOCK_MONOTONIC, &ts);
diff_timespec(&rs, &ts, &ref_time);

+ perf_stat__reset_shadow_per_stat(&rt_stat);
read_counters(&rs);

if (STAT_RECORD) {
--
2.21.1